OCT is a technique wherein imaging information can be obtained in the depth or z-direction of a sample, typically the retina of the eye. In one type of OCT, the retina is scanned with an object beam from an interferometer having a broadband light source with a short coherence length. A signal is obtained from the returned beam at depth positions wherein the optical path difference is less than the coherence length, which is typically in the order of a few microns. By adjusting the path length in the reference beam, it is possible to adjust the depth position within the target, which is assumed to be partially transparent, from which imaging information is obtained. In order to obtain a useful signal, as is known in the art, some form of modulation must be applied to the object beam.
Different scanning techniques may be employed as described, for example, in U.S. Pat. No. 5,975,697, the contents of which are herein incorporated by reference. In the so-called A scan, the sample is scanned along a single axis in the depth direction to generate a reflectivity profile along the Z axis at a particular point in the X-Y plane. In a B scan, the sample is also scanned in either the X or Y direction so as to generate a horizontal or vertical slice extending into the sample. The B-scan results from a succession of A scans. In en-face scanning, with which the present invention is concerned, image slices in the X-Y plane are taken at different depths to build up a composite three-dimensional image of the object. This is achieved by varying the optical path difference between the reference beam and object beam in the interferometer. A displaceable mirror is placed in the reference beam to vary the path length of the reference beam and thereby the optical path difference.
In a conventional OCT arrangement, the returned light from the object beam is mixed with the reference beam in a coupler and passed through a splitter to a differential photodetector arrangement. As the object beam scans across the target in the X-direction in a raster fashion, the instantaneous signal at the output of the photodetector arrangement is a function of the reflectivity of the target at each pixel at the current coordinate position determined by the scanner and at a depth determined by the optical path difference, which in turn is set by a displaceable mirror in the path of the reference beam. At the end of each frame, corresponding to a complete raster scan, the position of the mirror is adjusted to change the depth position, i.e. the position on the Z axis from where image information is obtained, and a new raster scan is performed at the new depth position determined by the position of the displaceable mirror. Multiple frames have to be taken in order to build up a three dimensional image of the target, or in the case of a single section image, multiple scans are required. The prior art in effect processes the signal pixel by pixel in a serial fashion. This is a slow and cumbersome process.